Here is what i found about this. Straight forward and not a big fix needed. The message ' the required file cannot be found' is referring to the song that it is trying to transfer to the iPod. Try shutting iTunes down and reopen it and sync it again.
1-Try and transfer again and look in the iTunes window at the song that is trying to transfer. Then go into the main library and uncheck that song. Then continue to update the iPod. Do this each time you get that error message when syncing the iPod.
2-Make sure the song that is trying to transfer to the iPod is in the iTunes folder. Check and make sure.
3-If it is then when done syncing the iPod re-import the song(s) into iTunes.
When that message appears when syncing the iPod the problem is the song(s) & iTunes cannot locate it/them to transfer to the iPod.
Good luck and let us know when you are jamming again :)